Hey I m trying to test that a suspending function is invoked

Question

Hey I m trying to test that a suspending function is invoked N times Every 30 seconds this coroutine should invoke the same suspending function

masteramyx · Answer

```ViewModel Make request every 30sec fun startRepeatingCall viewModelScope launch <http Dispatchers IO|Dispatchers IO> while true fetchFromNetwork delay 30000 private suspend fun fetchFromNetwork runCatching repository fetchFromNetwork ``` ```class TestVmClass MockK private lateinit var repository private lateinit var viewModel private val scheduler = TestCoroutineScheduler private val testDispatcher = StandardTestDispatcher scheduler Before fun setup Dispatchers setMain testDispatcher Test fun `test repeating task` = runTest testDispatcher viewModel startRepeatingCall tried different ways of advancing virtual clock coVerify exactly = 3 repository fetchFromNetwork ```

masteramyx · Answer

My goal was to call the viewmodel function advance time by 90 seconds and assert that it s been called 3 times The method is only called once though it seems like I m unable to synchronize the test coroutine context with the one used in my viewmodel I thought that by setting the test dispatcher my `viewModelScope` would use this dispatcher and when I call `advanceTimeBy x ` the time within that `viewModelScope launch ` block would respect that Thus the `delay` would reach it s end point and my `fetchFromNetwork ` would be called again

Patrick Steiger · Answer

I suggest to not use `viewModelScope` and instead you can e g pass in a `CoroutineScope` that is a child of the scope created by `runTest` to your ViewModel constructor `ViewModel` base class takes in a vararg of `Closeable` so you can pass CoroutineScope as a Closeable to it so it auto closes the scope on `onCleared `

Patrick Steiger · Answer

Or take in a dispatcher in constructor and do ` viewModelScope + dispatcher ` to run your ViewModel coroutines I prefer the previous suggestion because it also sets the Job so structured concurrency with `runTest` job is respected

masteramyx · Answer

I ve never seen the 1st approach you re suggesting One problem is that I m using `AndroidViewModel` so i don t believe we have that constructor available What I don t like about 2nd approach although it works Is that I have to expose the new `val scope = viewModelScope + dispatcher ` because in my tests I m unable to cancel the coroutines running within that scope without access to it For example ``` Test fun `test polling every 30 seconds` = runTest coEvery repository fetchFromNetwork coAnswers Response println Virtual Clock Before $ this currentTime viewModel startRepeatingTask advanceTimeBy 90000 println Virtual Clock After $ this currentTime viewModel scope cancel I can t cancel coroutine running inside VM with `TestScope` or `TestDispatcher` but only with scope those coroutines were launched in coVerify exactly = 3 repository fetchFromNetwork ``` It makes sense that you could only cancel a coroutine from scope it was launched in I guess I could also have one of these VM functions return a job which then I could have a reference to and cancel

Matt Rea · Answer

Patrick is correct that s the best way to make your ViewModel coroutines testable Note that the vararg Closeable in the ViewModel constructor is only available on Lifecycle version 2 5 and up <https developer android com topic libraries architecture viewmodel clear dependencies>